The present invention relates to the field of thin film transistor array driver circuits used to drive active matrix liquid crystal display devices and their manufacturing methods. More particularly, the present invention relates to thin film transistor arrays for driving liquid crystal display devices that include a charge capacitor.
Display devices using liquid crystal achieve low power consumption and reduced weight, which represent considerable improvements on conventional CRT displays. In particular, active matrix liquid crystal display devices using thin film transistors (TFTs), acting as a switching device for each pixel, are extensively used in displays for notebook PCs and car navigation systems due to their advantages of sharp images with less crosstalk. The use of these active matrix liquid crystal display devices is rapidly extending to large display monitors.
A conventional circuit containing a TFT array for driving this type of liquid crystal display devices is described next with reference to drawings. FIG. 3 is a schematic diagram of an example of a conventional TFT array in a liquid crystal display device. FIG. 4 is a sectional view taken along the broken line 4xe2x80x944 in FIG. 3. In this example, as shown in FIGS. 3 and 4, scanning lines 2 and video signal lines 5 are disposed in a matrix on a glass substrate 1. A pixel electrode 7, TFT 13, and storage capacitor 14 are disposed in a region surrounded by the scanning lines and video signal lines. The TFT 13 is mainly made of an amorphous silicon semiconductor layer 4. The TFT drain electrode 11 and the pixel electrode 7 are connected through a contact pattern 8. The video signal line 5 also acts as the source electrode of the TFT 13. The storage capacitor 14 is formed between an upper electrode 10 and the scanning line 2 so as to sandwich the first insulating film 3 which becomes a gate insulator. A second insulating layer 6 is formed as a passivation insulating layer for protecting the TFT 13.
Operation of the liquid crystal display device using the TFT array as configured above is described next.
First, when voltage is applied to the scanning line 2, which is the TFT gate electrode, a channel is established in the amorphous silicon semiconductor layer 4 of the TFT 13. Then, video signals from the video signal line 5 are fed to the drain electrode 11 through the TFT channel, and reach the pixel electrode 7 to change the orientation of the liquid crystal (not illustrated) held between the pixel electrode 7 and counter electrode (not illustrated) to the intended degree by a magnetic field established between the pixel electrode 7 and the counter electrode facing the pixel electrode 7. This adjusts the light transmittance to produce the required images.
In general, a storage capacitor is provided to the pixel electrode for maintaining the pixel electrode potential until the scanning signals are applied for the next frame. With the increased size of monitor displays, formation of a uniform electric capacitance in this storage capacitor is becoming a key factor in reducing defects such as uneven luminance and improving image quality of uniformity.
TFT arrays as described above are generally manufactured by means of the next key steps. Midway steps in the manufacturing process are described next with reference to FIGS. 6A to 6D. FIGS. 6A to 6D are sectional views taken along the broken line 6xe2x80x946 in FIG. 3, with the left part illustrating the storage capacitor 14 and the right part illustrating the TFT 13.
A first metal layer is formed on the glass substrate 1, and the metal is selectively etched to form the pattern of the scanning lines 2. Then, the gate insulator 3, which is the first insulating layer, is formed. The semiconductor layer is next formed, and the pattern of the TFT channel 4 is etched (FIG. 6A). Next, a second metal layer is formed, and this is selectively etched to simultaneously form patterns of the video signal line 5, drain electrode 11, and upper electrode 10 on the storage capacitor (FIG. 6B). The second insulating layer 6, which is the TFT passivation layer, is next formed and this layer is selectively etched to create openings 8 and 12 respectively on the drain electrode 11 and the upper electrode 10 of the storage capacitor (FIG. 6C). Finally, a transparent conductive layer is formed, and this is selectively etched to form the pattern of the pixel electrode 7.
With this conventional configuration, however, the exposed gate insulator 3 is often over-etched during the step of etching the channel 4 of the TFT 13, as is clear from FIG. 6A. As a result, the thickness of the remaining gate insulator 3 becomes non-uniform, also causing non-uniform thickness in a dielectric layer of the storage capacitor 14. This results in variations in capacitance of the storage capacitor 14 among pixels, leading to image defects such as uneven luminance.
The present invention aims to offer a thin film transistor array and its manufacturing method for achieving a uniform electric capacitance for the storage capacitors and reducing defects such as uneven luminance so as to improve image uniformity. It is therefore an object of this invention to provide a driving circuit which may be used, inter alia, for driving a display device comprising:
a thin film transistor
a pixel electrode electrically connected to said thin film transistor
a scanning line connected to the thin film transistor;
an insulating layer over said scanning line;
a semiconductor layer over a portion of the insulating layer and the scanning line and
a storage capacitor comprising an upper capacitor electrode formed over the semiconductor layer electrically connected to said pixel electrode.
Another object of this invention is a circuit for driving a display device comprising an array of a plurality of display elements each of the display elements comprising a pixel electrode, the circuit also comprising an array of scanning lines and an array of signal lines the signal lines arrayed substantially perpendicular to the scanning lines and electrically insulated therefrom, and a plurality of individual drivers, one driver for each display element, each of the individual drivers comprising:
a thin film transistor connected to a scanning line and a pixel electrode;
a storage capacitor electrically connected to the pixel electrode comprising an upper capacitor electrode, a dielectric layer and a lower electrode wherein the dielectric layer comprises an insulating layer over the lower electrode and a semiconductor layer over the insulating layer and under the upper electrode.
Still an object of this invention is a display device comprising an array of liquid crystal display elements each comprising a pixel electrode the display device further comprising an electronic circuit for driving the display device, the electronic circuit comprising an array of scanning lines and an array of signal lines the signal lines arrayed substantially perpendicular to the scanning lines and electrically insulated therefrom, and a plurality of individual drivers, one driver for each display element, each of said individual drivers comprising:
a thin film transistor connected to one of said scanning lines and said pixel electrode;
a storage capacitor also electrically connected to said pixel electrode, comprising an upper capacitor electrode, a lower electrode and a dielectric layer therebetween, wherein said dielectric layer comprises a first insulating layer over said lower electrode and a semiconductor layer over said first insulating layer and under said upper electrode.
Further, still in accordance with this invention, there is contemplated a method for manufacturing the thin film transistor array of the present invention that comprises the steps of forming a scanning line pattern on the insulated substrate; depositing the first insulating layer; forming a semiconductor island pattern for each TFT and storage capacitor by selective etching; forming patterns for video signal lines and an upper electrode for the storage capacitor by selective etching; and forming a pixel electrode pattern by selective etching.
Provision of the island pattern of the semiconductor layer between the upper electrode of the storage capacitor and the dielectric of the first insulating layer, which becomes the gate insulator, makes it possible to suppress any over-etching of the first insulating layer which acts as the dielectric of the storage capacitor during the step of forming the semiconductor island pattern. Accordingly, the dielectric layer of the storage capacitor is uniformly formed, reducing typical defects of liquid crystal display devices such as uneven luminance, and thus improving the uniformity of the images.
The method for forming a driving circuit for driving a display device when the driving circuit comprises a thin film transistor and a charge capacitor, comprises:
forming on a substrate a first conductive layer
selectively etching said first conductive layer to form at least one scanning line;
forming a first insulating layer over said etched conductive layer and substrate
forming a semiconductor layer over said first insulating layer selectively etching said semiconductor layer to form a thin film transistor channel and a protective island over said scanning line, said scanning line forming a gate electrode for said thin film transistor;
forming a second conductive layer over said etched semiconductive layer and said first insulating layer; and
selectively etching said second conductive layer to form an upper capacitor electrode for said charge capacitor over said semiconductor island, the scanning line forming a lower electrode for said charge capacitor, and to form a drain and a source electrodes for said thin film transistor.